Current treatment of older children and adolescents with hyperphenylalaninemia (HPA) is unsatisfactory because available elemental medical foods (EMFs) are not consumed in recommended amounts due to unpleasant organoleptic (taste) qualities. The long-term objective of this proposal is to improve the nutritional treatment and biochemical control of patients with HPA. Specific aims are to develop and test for safety and efficacy a new EMF (the Oregon EMF.) The Oregon EMF is designed to achieve the following: 1) reduce EMF protein: calorie density; 2) alter levels of individual amino acids (AAs) to meet Recommended Dietary Allowances (RDAs); 3) diminish levels of unpleasant-tasting AAs; and 4) synthesize and taste-test derivatives of the unpleasant-tasting, indispensable AA, methionine. Clinical safety and metabolic efficacy of the Oregon EMF will be determined in 15-20 HPA patients six years and older. Physical, psychological and nutritional assessments and laboratory data will be collected according to The Guidelines for Clinical Evaluation of New Products Used in the Dietary Management of Metabolic Disorders, Committee on Nutrition, American Academy of Pediatrics. Additional measures will be used to determine acceptance of the Oregon EMF, mealtime behaviors and school progress.